1. Field of the Invention
The present invention relates to a process for the thermal photoswitching of spin-transition compounds and to the use of such a process in the temporary or permanent marking of materials comprising particles of at least one spin-transition compound.
2. Description of Related Art
Compounds involving the iron(II) ion based on a triazole ligand, and the materials incorporating them, are known to exhibit the phenomenon of spin transition, combining a memory effect (thermal hysteresis in the vicinity of ambient temperature) with a modification to the optical (changing color), magnetic and structural properties. These properties are taken advantage of in use of its materials in various applications, in particular in information storage. Such compounds can be coordination complexes comprising one or more metal centers having a 3d4, 3d6 or 3d7 configuration, one or more nitrogenous ligands and one or more anions, such as described, for example, in patent applications EP-0 543 465, EP-0 666 561, EP-0 745 986 and EP-0 842 988, international application WO2007/065996 and patent application FR 2 917 410.
By way of example, patent application EP-0 543 465 describes iron-based spin-transition compounds and their use in information storage. Some of these compounds correspond to one of the following formulae:                FeL3(NO3)2 in which L is a ligand of the 1,2,4-triazole or 4-amino-1,2,4-triazole type combined with the NO3− anion;        Fe(ATP)2.5Cl2, in which the ATP ligand is 4-amino-1,2,4-triazole combined with Fe(II) and with Cl−;        Fe(TP)2Cl2, in which the TP ligand is 1,2,4-triazole, combined with Cl−;        [Fe(2-aminomethylpyridine)3]Cl2EtOH, EtOH being ethanol;        [Fe(1,10-phenanthroline)2](NCS)2;        [Fe(1-propyltetrazole)6](BF4)2.        
With the exception of [Fe(1,10-phenanthroline)2(NCS)2], these compounds are pink in the low spin (LS) state and white in the high spin (HS) state.
International application WO2007/065996 describes a material in the form of complex nanoparticles corresponding to the following formula (I):
                                          [                                                            (                                                            Fe                                              1                        -                        y                                                              ⁢                                          M                      y                                        ⁢                                          L                      3                                                        )                                w                            ⁢                              L                3                                      ]                    ⁡                      [                                          X                                  2                  /                                      x                    ⁡                                          (                                              1                        -                                                  z                          /                                                      x                            ′                                                                                              )                                                                      ′                            ⁢                              Y                                  2                  ⁢                                      z                    /                                          x                      ′                                                                      ′                                      ]                          w                            (        I        )            in which:                L represents a 1,2,4-triazole ligand carrying an R substituent on the nitrogen in the 4 position;        X′ is an anion having the valency x, 1≦x≦2;        Y′ is an anion other than X having the valency x′, 1≦x′≦2;        R is an alkyl group or an R1R2N— group in which R1 and R2 each represent, independently of the other, H or an alkyl radical;        M is a metal having a 3d4, 3d5, 3d6 or 3d7 configuration, other than Fe;        0≦y≦1, 0≦z≦2 and 3≦w≦1500.        
The compounds described in this document are pink in the LS state and white in the HS state.
Patent application FR 2 917 410 describes a spin-transition material composed of at least one compound corresponding to the following formula (II):AXbYc  (II)
in which:                A corresponds to the formula Fe1−mMm(R-Trz)3;        M is a metal having a 3d4, 3d5, 3d6 or 3d7 configuration, other than Fe;        0≦m≦1;        R-Trz represents a 1,2,4-triazole ligand carrying an R substituent on the nitrogen in the 4 position;        R is an alkyl group or an R1R2N— group in which R1 and R2 each represent, independently of the other, H or an alkyl radical;        X represents at least one monovalent or divalent anion;        Y represents at least one anion which has a chromophore group;        b and c are chosen so that the electrical neutrality of the compound (II) is respected. The color of each spin state of this material can be adjusted by appropriately choosing the X and Y anion or anions and by controlling their respective proportions in the material.        
The document FR 2 755 696 describes a spin-transition compound including a network comprising molecules each formed of a metal-ligand complex and of an anion, in which the metal is composed of at least one metal ion having a d4, d5, d6 or d7 electronic configuration, in which the ligand comprises at least one aminotriazole group or a substituted aminotriazole group, and in which the anion is formed of an alloy of at least two anions including an anion having a nitrate (NO3−) radical. This document also describes a data display device comprising a system forming a screen and a thermal addressing system. This thermal addressing system comprises heating means B1 (such as a laser beam with a wavelength in the infrared region). The function of the laser beam is thus not to switch the abovementioned compound but to monitor the photoswitching phenomenon.
Several methods which make it possible to bring about the spin transition of these compounds, that is to say which make it possible to change these compounds from an LS state to an HS state, have already been provided.
According to EP-0 543 465 and FR 2 917 419, the spin transition is brought about by heating or cooling and takes place between −20° C. and +100° C. The hysteresis phenomenon can range from a few degrees to a few tens of degrees, according to the compounds.
Some authors, such as Freysz et al. (Chemical Physics Letters, 2004, 394, 318-323), provide for the application of laser radiation at the center of the thermal hysteresis loop of iron-based spin-transition materials ([Fe(PM-BiA)2(NCS)2] complex with PM-BiA=N-(2′-pyridylmethylene)-4-aminobiphenyl), in order to bring about photoconversion from the LS state to the HS state. According to this document, irradiation is carried out for a few minutes at a wavelength of 830 nm (near infrared), at a power of 5 mW·cm−2 and at a temperature of 10 K (i.e., approximately −263.15° C.). Above a temperature of −100° C., photoswitching according to this process is no longer observed.
Other authors, such as Bonhommeau S. et al. (Angew. Chem. Int., 2005, 44, 4069), report a single laser irradiation test on a spin-transition material of [Fe(pyrazine){Pt(CN)4}] type in the dehydrated state. According to this document, irradiation is carried out in the visible region at a wavelength of 532 nm with pulses having a duration of 8 ns. However, there are a number of disadvantages to this technique. It requires the use of a nanosecond laser pulse having an energy of several tens of millijoules. In addition the central wavelength of the pulse has to be within the absorption range of the material.
These various laser irradiation methods used to date are based on the LIESST (Light-Induced Excited Spin State Trapping) effect, which is an electron photoexcitation of the metal ions, such as the iron ions, of the spin-transition complexes.
The major disadvantage of these methods is that they have to be carried out at extremely low temperatures (of the order of approximately −260° C.) and that they do not make possible the photoswitching of the spin-transition compounds when they are applied at ambient temperature, which is a major limitation on the application of these processes in industry.
The scientific publication by Gallé et al. (“Room temperature study of the optical switching of a spin crossover compound inside its thermal hysteresis loop”, Applied Physics Letters, AIP, AMERICAN INSTITUTE OF PHYSICS, vol. 96, No. 4, of Jan. 27, 2010, pages 41907-1 to 41907-3) describes a process for the photoswitching of a spin-transition material from the low spin state to the high spin state comprising a stage of exposure of said material to pulsed laser radiation at a wavelength of between 0.355 μm and 0.532 μm, at ambient temperature and at a power of 52 mJ·cm−2 for 6 ns, i.e. 8.66×10+6 W/cm−2. However, this process exhibits the disadvantage of requiring considerable power and of consequently being expensive to carry out.